As well known to those skilled in the art, several types of toothbrushes, each of which is used for cleaning human teeth and maintaining dental hygiene, are proposed and widely used. Most typical toothbrushes are individually provided with a long handle. A great number of nylon bristles are densely set on one end of the handle, thus forming a brushing head of the toothbrush. A replaceable toothbrush, of which the brushing head is detachably attached to the handle thus being replaceable with a new head when existing bristles are frictionally abraded or deformed, is proposed.
In most typical toothbrushes, the bristles are made from synthetic fiber. For dental hygiene, most dentists advise people to brush one's teeth three times a day within three minutes after breakfast, lunch and dinner. Such repeated tooth brushing action allows the synthetic fiber bristles to injure one's teeth or gums, thus causing the teeth to be sensitive to cold while eating or drinking cold or sour food or beverages. A problem experienced in a known toothbrush with a replaceable head is that the brushing head fails to be securely fixed to the handle and is unexpectedly separated from the handle. The nylon bristles are sharpened at their tips so that the bristles are not free from injuring one's teeth.
Meanwhile, toothbrushes with animal hair bristles. which were typically used at the time of the undeveloped petrochemical industry, do not injure one's teeth or gums. However, the animal hair bristles are problematic in that they are easily removed from the handles.
The animal hair bristles are also expensive and have a low durability so that such bristles increase production costs of the toothbrushes and reduce the expected life span of the toothbrushes. In this regard, the bristles of toothbrushes have been made from synthetic fiber, which is easily prepared and has a high durability.
While a user brushes his teeth using such a toothbrush with the synthetic fiber bristles, the sharpened tips of the bristles physically crush the food remnants stuck on the teeth or lodged between the teeth and remove the food remnants while polishing the teeth.
However, the synthetic fiber bristles may injure one's teeth or gums while brushing the teeth and may cause a rejection symptom of the teeth because the synthetic fiber does not have an affinity for the human body including teeth.
The damage to the teeth caused by such synthetic fiber bristles is more serious than that expected from an organic material such as cloth, leather or fur having a hardness similar to the synthetic fiber bristles.
It is thus preferable to produce the bristles of a toothbrush using organic materials having an affinity for the human body. However, the bristles made from such organic materials are problematic in that they fail to achieve the tooth cleaning effect expected from the synthetic fiber bristles. It is thus necessary to produce a toothbrush using a material, which is not expensive and easily handled during a production process and is free from generating toxic materials while brushing teeth, and has a high durability.
In order to provide an appropriate material for toothbrushes, physical and chemical characteristics of known materials have been actively studied by the applicant of this invention as follows.
A) Crude rubber (natural rubber): crude rubber is typically produced by adding an acid into latex (rubber tree sap) prior to performing a congelation and drying process. Known crude rubbers are classified into various types in accordance with manufacturing process and quality. However, the known crude rubbers are problematic in that they individually include volatile materials laden with a very small amount of impurities such as ash, copper, manganese and nitrogen.
B) Special crude rubber: this rubber is specifically produced from natural rubber in accordance with the use of the resulting special rubber. However, such a special crude rubber has the same problem experienced in natural rubber.
C) Styrene rubber: styrene rubber has been most widely used as it makes up at least 80 percent of marketed and practically used rubbers. This rubber effectively resists aging, fire and frictional abrasion so that it is preferably used for producing tires.
D) Butadien rubber: the amount of gel and ash included in butadien rubber is smaller than that of the other rubbers. However, it is not known whether butadien rubber is harmful to the human body or not.
E) Chloropren rubber: this rubber is chemically manufactured and used for various industrial applications. However, it is not known whether chloropren rubber is harmful to the human body or not.
In addition, a rubber, which is chemically produced and has the same structural formula as natural rubber, is known. However, this rubber is problematic in that it includes poisonous components and has synthetic rubber odor irritating to people.
Other rubbers, such as butyl rubber, nitrile rubber, ethylenepropylene rubber, acryl rubber, urethane rubber, silicon rubber or fluoric rubber, are known.
The above rubbers, except for the rubbers of which harmfulness to the human body is not known, were subjected to several tests, measuring an affinity for the human body, thermal resistance, low temperature resistance, electric characteristics and consume resistance. As a result of the above tests, silicon rubber is known as the most preferable material for the bristles of a toothbrush, irrespective of expensiveness.
Silicon rubber has a coil structure with a small intermolecular force so that it has high elasticity and compressibility and more preferably resists low temperature in comparison with the other rubbers. Due to the free rotation of the methyl group on the outside of the coil structure, silicon rubber has a unique surface action such as water repellency and separation power, thus being most preferably used in water.
Silicon rubber has the following physical and chemical characteristics.
1. Thermal resistance: silicon rubber is not thermally affected at 150.degree. C. and effectively endures for at least 10,000 hours at 200.degree. C. and at least 1 hour at 350.degree. C. When the silicon rubber is burnt at a high temperature of not less than 500.degree. C. in air, siloxane polymer is cut, causing silica (white silicon) to remain exclusively.
2. Low temperature resistance: silicon rubber loses its elasticity at temperatures of -60 to -70.degree. C., while the other organic rubbers lose their elasticity at temperatures of -20 to -30.degree. C. It is thus known that silicon rubber effectively maintains elasticity at a low temperature at which the other rubbers lose elasticity. Furthermore, a silicon rubber, which effectively maintains its elasticity at an extremely low temperature of about -100.degree. C., is known.
3. Weather resistance: silicon rubber is free from deterioration caused by ozone generated from corona discharge, while the other rubbers are seriously deteriorated by such ozone and lose their desirable characteristics. Silicon rubber is not deteriorated even when it is exposed to ultraviolet rays, wind or rain for a lengthy period of time. This means that a toothbrush made from silicon rubber is not ill-affected by water, saliva or toothpaste.
4. Electric characteristics: silicon rubber has a high electric insulation (10.sup.14 -10.sup.16 .OMEGA./m) and is stable both in a wide temperature range and a wide frequency range. Silicon rubber effectively maintains its electric characteristics in water since it has a high water repellency. Furthermore, silicon rubber effectively maintains electric insulation even on corona or arc discharge.
5. Thermal conductivity: thermal conductivity (0.5.times.10.sup.3 cal/cm.multidot.sec.multidot..degree. C.) of silicon rubber is relatively higher than that of the other rubbers so that a user does not feel heat or coldness from silicon rubber even when the rubber is used at a high or low temperature. In this regard, the silicon rubber may be preferably used at a high or low temperature. The thermal conductivity of silicon rubber may be increased or reduced by appropriately selecting a filler or changing the mixing ratio of the filler during a process of forming the silicon rubber.
6. Moisture resistance: silicon rubber is almost free from impregnation of water (not higher than 1 wt %) even when it is immersed in cold, hot or boiled water for a lengthy period of time. The silicon rubber is not affected by steam. Moisture resistance of the silicon rubber may be further improved by appropriately selecting a vulcanizing agent or carrying out a secondary vulcanization during a process of producing the rubber. Due to such a high moisture resistance, silicon rubber does not become wet under any circumstance.
7. Chemical resistance: silicon rubber has a high chemical resistance so that it is not changed physically or chemically in aniline or alcohol and effectively resists diluted acid or alkali. In a nonpolar organic compound (solvent) such as toluene, gasoline or benzene, the silicon rubber slightly expands in volume (10-15%), but does not change its characteristics. When such a solvent is removed from the silicon rubber, the rubber is restored to its original volume.
8. Resistance to compressive distortion and bending fatigue: silicon rubber has a high resistance to compressive distortion and bending fatigue so that the rubber does not permanently change its configuration at any state but is restored to its original configuration when an external force is removed. Therefore, the bristles of a toothbrush made from silicon rubber are not deformed even when the toothbrush is use for a lengthy period of time. The toothbrush can be thus effectively used until the silicon rubber bristles are frictionally consumed, losing the tooth cleaning effect. It is possible to prepare silicon rubber of a special grade with bending fatigue resistance being increased by 8 to 20 times in comparison with general silicon rubber.
9. Tensile strength: silicon rubber typically has a tensile strength of 14.7 kN/m which is higher than that of other rubbers. The tensile strength of silicon rubber may be increased by 2 or 3 times when a filler or vulcanizing agent is appropriately added to the rubber. Therefore, the silicon rubber bristles are almost completely free from being unexpectedly severed.
10. Fire retardancy: silicon rubber rarely catches fire and is rarely extinguished once it catches fire The fire retardancy of the silicon rubber may be increased by the addition of a small amount of fire retarding agent.
11. Physical inactivity: silicon rubber is physically inactive so that it has an affinity for a living body and rarely causes a rejection symptom in the living body. When the lid of a live vaccine container is made from silicon rubber, it is possible to keep a live vaccine for a lengthy period of time. However, the lid made from another material regrettably causes a live vaccine to be unusable in a short time. In addition, blood rarely congeals on silicon rubber. In this regard, the silicon rubber is preferably used as a material for medical rubber lids, ultrasonic-diagnostic lens, endoscopic hose, catheter, and artificial internal organs. Recently, silicon rubber is also used as an insert for the human body such as an artificial nose bone or artificial breast. The silicon rubber for medical applications is formed into a rubber of a special grade. The typical silicon rubber, which is not colored or impregnated with impurities on purpose, exclusively includes Si--O and is completely transparent. Silicon rubber effectively and selectively allows gas or vapor to pass through so that the rubber is preferably used for medical applications, for example, artificial hearts, artificial lungs and incubators.
12. Inadhesive and anticorrosive power: silicon rubber is inadhesive and highly releasable since it is a stable and chemically inactive compound. Therefore, silicon rubber does not combine with another material, thus being free from self-corrosion or corroding another material. Silicon rubber is preferably used as a material for the fixing roller of a copying machine, a roll of a printer or a roast wax. The silicon rubber bristles of a toothbrush do not allow food remnants to be stuck to the bristles, thus effectively preventing the propagation of bacteria on the toothbrush.